Field of the Invention
The present invention relates to a vehicle braking unit, of the type comprising at least one spring axially actuating the brake, such as a helical spring maintained in the brake release position against its reaction force through a screw-nut system adapted to be driven in rotation by an electric motor the electrical feeding current of which is adapted to adjust the braking force of the unit, constituted by the axial reaction force of the spring reduced from the subtractive force provoked by the screw-nut system rotatively driven by the electric motor. The invention also concerns the application of this braking unit to a service park, and/or emergency braking system fitted, where necessary, with anti-skid means.
In order to dispose of a spring-brake of which the release does not require the use of a pressurized fluid such as compressed air or pressurized oil, braking units have already been proposed and produced in which the brake application force is supplied by a prestressed spring such as a helical spring and the adjustable release effort is performed by an electric motor which delivers, for example, an axial release effort via a screw presenting reversibility characteristics, such as a ball screw (ball circulation worm gearing). In such a braking unit, the motor which is generally a direct current motor, with separated and constant excitation by permanent magnets, is maintained under full voltage in brake release position. The progressive brake application is achieved through progressive decrease of the motor voltage, i.e. according to the most widely used adjustment method, of the direct current flowing through the rotor windings of the motor.
The brake application force is thus the result of the difference between on the one hand, the reaction force of the spring considered as constant but which decreases slightly with the return stroke of the spring, and theoretically slightly decreases in time as a consequence of a relaxation effect of the metal of the spring and, on the other hand, the adjustable release force (through adjustment of the motor current), transmitted by the ball screw which presents a restricted and acceptable hysteresis rate of about 10%.
These braking units which present the advantages of spring brakes with respect to operating security due to the fact that it is always possible to interrupt the release effort and has the facility of placing in long term park position through cutting out of the release current, are used on vehicles where, for reasons of bulkiness ad facility of maintenance, it is desired to suppress the need of a pressurized fluid source for the braking, while changing to "all electric" piloting of the brakes. However, the braking units must be placed immediately adjacent to the braking surfaces upon which they act and which are constituted generally by the lateral faces of a brake disc and the important volume of electric motor, spring and slack adjusting means between the braking surfaces and the brake linings generally lead to a bulky assembly which is very difficult to house within the bogie chassis of light railway stock such as street cars, or rapid vehicles such as high speed trains, which are more adapted to receive high security braking units with "all electric piloting".
One of the objects of the present invention is specifically to offer a braking unit which allows to retain the qualities of the spring brake released by an electric motor and which can be housed more easily in the chassis or the bogie of the vehicle while acting simultaneously on several braking surfaces such as those of multiple discs brake of a high-speed railroad bogie.